A unit used to aggro (usually-ranged) enemies so that your own ranged units don't get attacked and your healers can focus on healing the lightning rod instead of frantically healing everyone left and right. Valkyries, princesses, and angels are popular choices for lightning rods due to their high MR. Bandits work because of their high HP too. Immortal Karma is basically the ultimate 13-second lightning rod for obvious reasons.